In today's world, there is a growing requirement for transferring different types of information over various distances via communications links. This information can be, for example, data generated by a computer, imaging data from a video camera, or any other of a number of different forms of data. This data often has to share the communications link with voice circuits that carry human conversations.
In modern communication systems, the carrying of multiple conversations over a common link is most frequently accomplished by the well-known methods of time division multiplexing (TDM) or frequency division multiplexing (FDM). One common multiplexing system is the American T1 system which has a link of 1.544 Mbit/sec that is divided into 24 slots of 64 Kbit/sec. This division is implemented by continuously sending "frames" of 192 bits. Each frame is divided into 24 slots, so that each slot has 8 bits. Since each slot can carry a different conversation, 24 different conversations can be carried simultaneously over the T1 system link, with 8 bits of each conversation transmitted every frame.
The different types of information (voice, data, image, etc.) that can be carried over a link have different transmission characteristics and requirements. For example, voice information (a conversation) is characterized by low to medium bandwidth and low latency. Latency is the period of time between the occurrence of an event and the time at which the event is recognized, while bandwidth is defined here as the bits transmitted per second. (Since a defined number of frames are transmitted per second, the number of bits allocated within a single frame will also be referred to as bandwidth). By contrast, computer data can have low to high bandwidth and relatively high latency. Image data can have extremely high bandwidth and low to high latencies.
As stated above, the different types of information must often be carried over the same link. There is, for example, at least one proposed system that provides an integrated network that carries both data and voice information over the same link. (For ease of description, all other types of information besides voice, such as computer data, image information, etc., will be henceforth referred to as "data".) This network is the Integrated Services Digital Network (ISDN), proposed as an industry standard.
The ISDN network allows both voice and data to be carried over the same link, with voice occupying some slots, and data occupying other slots, with at least one slot acting as a control slot. The information carried in the control slot tells the receiver how the slots in each frame are allocated, i.e., which slots are assigned to which receivers at any particular time. An example follows, with reference to FIG. 1.
The 24 slots of a frame carried by the ISDN system is shown in FIG. 1. The first slot is a slot for control, and is thus the control channel. The remaining 23 slots can be assigned to 23 different, simultaneous transfers of information. Each separate transfer of information is labeled a channel, so that the ISDN system can support up to 23 separate channels, and one control channel. However, each channel can occupy one or more contiguous slots.
The ISDN system suffers from a number of drawbacks. One of these drawbacks is its inflexibility in its allocation of slots. For example, assume that one type of information transfer needs higher bandwidth than can be provided by one slot. In the ISDN system, this is accomplished by grouping together a number of contiguous slots, to form a channel having higher bandwidth since each frame is now allocated to carry more information for that particular information transfer.
The requirement that the slots be contiguous for information carried over the same channel leads to inefficient use of the link in the ISDN systems. This is because certain allocations can cause, for example, only single slots to be available when a channel needs two or more contiguous slots. In such a circumstance, either the channel waits for two contiguous slots to become free, or a re-allocation must be performed. In either case, the link is being inefficiently used.
Another drawback to the ISDN system is the relatively long time it takes to change the allocation scheme of the slots When the allocation scheme is to be changed, the sender sends the receiver the new allocation scheme. Upon receipt of the new allocation scheme, the receiver will then send back an acknowledgment signal. The sender starts sending information according to the new allocation scheme only when it receives the acknowledgment signal from the receiver. During this whole time period, no useful information can be sent because both sender and receiver cannot be sure that the receiver has the correct new allocation scheme until after the sender receives the acknowledgment signal. When the link spans the entire country so that the propagation of the signals takes a relatively long time (30 m sec, for example), the acknowledgment method of changing the allocation scheme is undesirably slow.
A need therefore exists for an integrated network that can carry both voice and data over the same link, this network providing flexibility in its allocation of slots. The network should also be able to switch the allocation scheme very quickly and transparently, so that information can be transferred during the switching process.